Rigid airships (Zeppelins) and blimps in the United States have used non-flammable helium rather than hydrogen as the lifting gas because of safety concerns, such as the Hindenburg disaster of the 1930's. However, such improvement in safety came at a substantial penalty in performance.
For example, the hydrogen-filled Zeppelins of the 1920's and 1930's regularly flew across the Atlantic from Friedrickshafen, Germany to Lakehurst, N.J. and Rio de Janeiro, Brazil. However, none of the helium-filled ships of this era had sufficient range for transoceanic flight. The German airship LZ-126 flew the 5,000 miles from Friedrickshafen to Lakehurst as a hydrogen ship. Renamed the Los Angeles and converted to helium by the U.S. Navy it could not make the 3,800 mile return flight to England, from Lakehurst to Cardington.
The inferior performance of helium-filled airships as compared to those using hydrogen as the lift gas is the result of three major factors. First, helium is simply less buoyant than hydrogen. At standard atmospheric conditions the buoyancy of hydrogen and helium are 71.2 and 66.0 pounds per thousand cubic feet, respectively: Thus the gross lift of a ship filled with hydrogen would be 8% more than if the same ship were filled with helium. The gain of net lift available for payload and fuel would be as much as 16%.
Second, and more importantly, the much higher cost of helium precludes routine valving off or venting lift gas during flight. Hydrogen airships were essentially completely (95%) filled with hydrogen gas for maximum lift at take off. Gas was then valved off during flight to maintain neutral buoyancy as fuel was consumed and to handle expansion of the gas due to increased altitude and decreased ambient pressure.
Helium ships by contrast were only 80-85% full at take-off to allow room for expansion of the helium caused by changes in altitude and temperature. Neutral buoyancy was maintained by condensing water from engine exhaust to offset the weight of fuel consumed. Since the helium airships were only partially full of lift gas at take-off such airships could not carry as much payload or fuel as hydrogen ships of the same size. The weight of the water recovery system and its fuel requirement further decreased the net pay load of helium airships.
Finally, the practical purity of helium lifting gas is less than that of hydrogen. Non-rigid airships are normally formed of fabric joined or seamed to form an aerostat when inflated with a lift gas, helium or hydrogen, at a slight positive pressure relative to the ambient air. Pressure of the airship is then controlled by internal air bags known as ballonets. Air is introduced or expelled from the ballonets to compensate for variations in the volume of the lift gas and thus maintains at all times a positive pressure on the aerostat, and its aerodynamic characteristics.
A particular operational problem with helium-filled airships and balloons is that their lift gas gradually becomes contaminated with air through leaks in the ballonet fabric or seals and by diffusion through (or punctures of) the main aerostat and ballonet fabrics. Thus, in normal operation, the helium lift gas in a helium airship gradually becomes contaminated with heavier oxygen and nitrogen, whereas with hydrogen filled airships the valving off of a substantial fraction of its lift gas during each flight and its replacement with pure hydrogen before the next flight kept air contamination to a very low level.
Typically in World War II blimps, the helium purity declined to less than 95% in about six months. At about this point the blimp had lost a substantial fraction of its gross lift and a much larger percentage of its net payload and it had to put into a maintenance base for helium replacement. At the base the ship was purged with purified helium to increase the purity up to 98 to 99%. The contaminated helium was purified and stored in high pressure cylinders for reuse. Smaller mobile purifiers were also developed which could withdraw a stream of contaminated helium, remove the impurities, and return the purified helium to the ship without intermediate storage. These units could purify the helium while the ship was in a hangar or moored to an external mast. On-board purifiers were not practical because of their weight and because continuous purification was not necessary or even desirable from an operational standpoint.
Helium may be purified in several other ways, including cryogenic distillation. In cryogenic distillation, everything other than helium is liquified or solidified and separated from the remaining helium gas. Low temperature adsorption on activated carbon was widely used by the Navy in World War II and is still used by some operators of advertising blimps.
More modern processes include the use of semi-permeable membranes and selective adsorption on molecular sieves to separate contaminants from the helium.
Semi-permeable membrane processes take advantage of the fact that helium passes through or permeates certain membranes many times faster than oxygen or nitrogen. In molecular sieve processes impurities are selectively adsorbed from helium in a manner similar to activated carbon, but at normal temperatures rather than the low temperatures required by the activated carbon methods. Both semi-permeable membrane and molecular sieve adsorption methods remove oxygen less efficiently than nitrogen.
It is accordingly a primary object of this invention to remove oxygen by a very efficient catalytic reaction process, thus substantially improving the efficiency of the overall helium purification process for either rigid or non-rigid airships or balloons.
During the 1950's the Navy developed heavy take-off techniques for its big blimps which took advantage of the aerodynamic lift generated by the envelope as the ship took off from a runway with a nose-up attitude. This allowed the Navy ZPG class airships to take off say 10,500 pounds heavy, something never attempted with the much longer rigid Zeppelins. This allowed the airships to take off with more fuel and greatly extended their range. However this technique requires take-off from a paved runway under favorable weather conditions and would not be applicable to routine refueling at sea or at unimproved land sites.
A further object of this invention is to temporarily increase the volume of helium by increasing its temperature to provide maximum lift when needed; for example, at take-off, or for special maximum-lift missions, without need of either such a paved runway or such favorable weather conditions. Particular advantage is taken of the fact that helium volume is directly proportional to its absolute temperature. For example, at 60.degree. F. (520.degree. R.) an increase in temperature of 52.degree. F. would increase the helium volume by 10%. This concept of heating or cooling the helium to cause it to expand or contract is known as "thermal buoyancy control" or a "thermal ballonet".
As airships gain altitude during flight the pressure decreases and the helium volume increases. In non-rigid blimp-type airships the increase in volume of helium is accommodated by venting air from a ballonet, a separate air compartment, so that the combined volume of helium plus air is always equal to the total volume of the aerostat. When all of the air has been released from the ballonet the airship is completely full of helium and the ship is at its pressure height. This is the maximum height that a blimp can reach and safely return to earth.
Typically a military blimp might have a ballonet volume equal to 30% of the main aerostat, giving it a pressure height of about 10,000 feet. If it goes above its pressure height to 15,000 feet, for example, the crew must valve off the additional volume of helium corresponding to the decrease in pressure between 10,000 and 15,000 feet (13%). This causes no immediate problem provided that the ship has ballast to release to compensate for the reduction in lift or can generate aerodynamic lift. However, when the ship returns to earth the process is reversed. As the helium volume decreases, air is blown into the ballonet to keep a small positive pressure of about two inches of water on the aerostat. The positive internal pressure keeps the aerostat fabric taut and maintains the aerodynamic shape of the aerostat necessary for smooth flow and minimum drag of the ship through the air.
As the descent continues, the ship having valved off a portion of its initial helium volume arrives at an elevation at which the ballonet is completely full of air and can not compensate for any further decrease in helium volume. If the descent continued, the pressure in the aerostat would fall below the surrounding air pressure, a dimple would form in the front of the aerostat, so that the aerodynamic drag would be greatly increased and the ship would be in grave danger under anything less than ideal weather conditions.
In accordance with the present invention, such a problem is overcome by heating the helium in the aerostat to increase its volume, thereby staying within the operational control range of the fixed volume ballonet. Accordingly another object of this invention is to extend the operating range of an airship to higher altitudes than would be permitted by its ballonet volume by providing additional volume compensation through the effect of temperature on lift gas volume.
Another problem associated with airships and balloons is that of snow and ice accumulation on the aerostat and control surfaces. Such aircraft are especially vulnerable because of their large surface area. For example, a quarter-inch of ice on a 30,000 square foot surface would add to more than eighteen tons of ice.
Known in-flight measures to cope with icing conditions include cycling the pressure of the airship above and below its normal pressure to convert the whole aerostat into a de-icing boot and constant exercising of the control surfaces to keep them from freezing up. Otherwise the standard operating procedure is to get into a hangar, if possible, and if not, to get airborne and seek an altitude above or below the icing or wet snow conditions. Accordingly the present invention provides methods and apparatus for substantially improving safety and operability of such airships or balloons under icing conditions either during flight or while moored.
A further object of this invention is to provide means either on board, or at mooring, to warm the helium quickly to warm the aerostat and control surfaces and thereby prevent the accumulation of ice or wet snow on the aerostat surfaces.